This invention relates in general to mechanical actuators and in particular to blade servoflap actuators for helicopters.
Helicopters offer many advantages over traditional fixed wing aircraft. Vertical rising capability allows the helicopter to operate in confined areas. Commercial operators use the helicopter for airport-to-city and interurban transportation, and the military uses it for transporting troops in and out of combat. Slow speed capability allows the helicopter to be used for forestry, pipeline and powerline patrols. Hovering capability allows the helicopter to be used as a rescue vehicle.
Vertical lift and forward propulsion are supplied by a rotor assembly whose mast is rotated about a vertical axis. The rotor is provided with a number of blades, designed as airfoils, which extend radially from the mast. When rotated in a horizontal plane by the mast, the blades accelerate air downward to provide a reactive lifting force, or they accelerate the air at an angle to provide lift and thrust. The direction in which the air is accelerated depends upon the pitch of the blades. Traditionally, blade pitch has been changed by hinging the roots of the blades to the rotor mast and twisting the blades about their longitudinal axes with a cyclic pitch changing device.
However blade pitch can also be changed by servoflaps located near the tips of the blades. In one servoflap system, the roots of compliant blades are rigidly attached to the rotor mast. Angular position of the servoflaps is changed by mechanical linkages extending through the blades and to the rotor mast. When the angular position of the servoflaps is changed, the air flowing thereover forces the blades to twist about their longitudinal axes, thereby changing blade pitch. This system was first disclosed by Charles Kaman in U.S. Pat. No. 2,455,866 issued on Dec. 7, 1948. This patent is incorporated herein by reference.
In another system, compliant blades are hinged at the mast in lead lag (back and forth) and flapping (up and down). As above, the change in angular position of the servoflaps causes blade pitch to change. Both systems provide smoother flight in practice.
It has been proposed to eliminate the mechanical linkages by mounting a rotary actuator at the tip of each blade. However, this arrangement is not feasible, particularly at high accelerations. At accelerations approaching 500 g's, lubrication is stripped away from the actuator's gearing, resulting in rapid wear and overheating of the actuator. High loads caused by rapid pitch changes further increase the wear and overheating